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id="article-container"><blockquote>
<p>LockSupport 是 JUC 中常用的一个工具类，主要作用是挂起和唤醒线程。在阅读 JUC 源码中经常看到，所以很有必要了解一下。</p>
</blockquote>
<h3 id="介绍"><a href="#介绍" class="headerlink" title="介绍"></a>介绍</h3><blockquote>
<p>基本线程阻塞原语创建锁和其他同步类。Basic thread blocking primitives for creating locks and other synchronization classes.</p>
</blockquote>
<blockquote>
<p>LockSupport 类每个使用它的线程关联一个许可（在意义上的Semaphore类）。 如果许可可用，调用 park 将立即返回，并在此过程中消费它; 否则可能阻塞。如果许可不是可用，可以调用 unpark 使得许可可用。（但与Semaphore不同，许可不能累积。最多有一个。）</p>
<p>方法 park 和 unpark 提供了阻塞的有效手段和解锁线程不会遇到死锁问题，而 Thread.suspend 和 Thread.resume 是不能用于这种目的：因为许可的存在，一个线程调用 park 和另一个线程试图 unpark 它之间的竞争将保持活性。 此外，如果调用者线程被中断，park 将返回，并且支持设置超时。 该 park 方法也可能返回在其他任何时间，“毫无理由”，因此通常必须在一个循环中调用的返回后重新检查条件。 在这个意义上park作为“忙碌等待”不会浪费太多的时间自旋的优化，但必须以配对 unpark 使用。</p>
<p>这三种形式的 park 还支持 blocker 对象参数。而线程被阻塞时是允许使用监测和诊断工具，以确定线程被阻塞的原因。（诊断工具可以使用getBlocker(Thread) 方法 。）同时推荐使用带有 blocker 参数的 park方法，通常做法是 blocker 被设置为 this 。</p>
</blockquote>
<p>上面的意思总结下来个人理解是：</p>
<ol>
<li>许可（permit）的上限是1，也就是说只有 0 或 1 。</li>
<li>park: 没有许可的时候，permit 为 0 ，调用 park 会阻塞；有许可的时候，permit 为 1 ， 调用 park 会扣除一个许可，然后返回。</li>
<li>unpark：没有许可的时候，permit 为 0 ，调用 unpark 会增加一个许可，因为许可上限是 1 ， 所以调用多次也只会为 1 个。</li>
<li>线程初始的时候是没有许可的。</li>
<li>park 的当前线程如果被中断，会立即返回，并不会抛出中断异常。</li>
<li>park 方法的调用一般要放在一个循环判断体里面。</li>
</ol>
<p>大概如图所示：</p>
<img src= "https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/loading.gif" data-lazy-src="https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/article/rGm1AX.png" align=center />


<p>下面是源码注释中的案例：</p>
<pre><code class="java">/**
 * FIFO 独占锁
 */
class FIFOMutex &#123;
   private final AtomicBoolean locked = new AtomicBoolean(false);
   private final Queue&lt;Thread&gt; waiters = new ConcurrentLinkedQueue&lt;Thread&gt;();

   public void lock() &#123;
     boolean wasInterrupted = false;
     Thread current = Thread.currentThread();
     waiters.add(current);

     // Block while not first in queue or cannot acquire lock
     // 不在队列头，或者锁被占用，则阻塞， 就是只有队列头的可以获得锁
     while (waiters.peek() != current || !locked.compareAndSet(false, true)) &#123;
       LockSupport.park(this);
       if (Thread.interrupted()) // ignore interrupts while waiting
         wasInterrupted = true;
     &#125;

     waiters.remove();
     if (wasInterrupted)          // reassert interrupt status on exit
       current.interrupt();
   &#125;

   public void unlock() &#123;
     locked.set(false);
     LockSupport.unpark(waiters.peek());
   &#125;
 &#125;
</code></pre>
<h3 id="验证"><a href="#验证" class="headerlink" title="验证"></a>验证</h3><h4 id="线程初始有没有许可？"><a href="#线程初始有没有许可？" class="headerlink" title="线程初始有没有许可？"></a>线程初始有没有许可？</h4><pre><code class="java">public class LockSupportTest &#123;

    public static void main(String[] args) &#123;

        System.out.println(&quot;开始执行……&quot;);

        LockSupport.park();

        System.out.println(&quot;LockSupport park 之后……&quot;);

    &#125;
&#125;</code></pre>
<ol>
<li>执行后会发现，代码在 park 处阻塞。说明，线程初始是没有许可的。</li>
</ol>
<h4 id="添加许可并消耗许可"><a href="#添加许可并消耗许可" class="headerlink" title="添加许可并消耗许可"></a>添加许可并消耗许可</h4><pre><code class="java">public class LockSupportTest &#123;

    public static void main(String[] args) &#123;

        System.out.println(&quot;开始执行……&quot;);

        LockSupport.unpark(Thread.currentThread());

        System.out.println(&quot;执行 - park&quot;);

        LockSupport.park();

        System.out.println(&quot;LockSupport park 之后……&quot;);

    &#125;

&#125;</code></pre>
<pre><code class="java">public class LockSupportTest &#123;

    public static void main(String[] args) throws InterruptedException &#123;

        Thread thread = new Thread(new Runnable() &#123;
            @Override
            public void run() &#123;
                System.out.println(&quot;线程 &quot; + Thread.currentThread().getName() + &quot;开始执行 park&quot;);
                LockSupport.park(this);
                System.out.println(&quot;线程 &quot; + Thread.currentThread().getName() + &quot;执行 park 结束&quot;);
            &#125;
        &#125;);

        thread.start();
        // 保证 上面线程先执行，然后再主线程
        Thread.sleep(5000);
        System.out.println(&quot;开始执行 unpark(thread)&quot;);
        LockSupport.unpark(thread);
        Thread.sleep(5000);
        System.out.println(&quot;执行 unpark(thread) 结束&quot;);

    &#125;

&#125;
</code></pre>
<p>通过上面示例可以看出：</p>
<ol>
<li>执行 unpark 可以进行给予指定线程一个证书。</li>
<li>线程当前被 park 阻塞，此时给予证书之后， park 会消耗证书，然后继续执行。</li>
</ol>
<h4 id="许可上限为-1"><a href="#许可上限为-1" class="headerlink" title="许可上限为 1"></a>许可上限为 1</h4><pre><code class="java">
public class LockSupportTest &#123;

    public static void main(String[] args) &#123;

        System.out.println(&quot;unpark 1次&quot;);
        LockSupport.unpark(Thread.currentThread());
        System.out.println(&quot;unpark 2次&quot;);
        LockSupport.unpark(Thread.currentThread());

        System.out.println(&quot;执行 - park 1 次&quot;);
        LockSupport.park();
        System.out.println(&quot;执行 - park 2 次&quot;);
        LockSupport.park();

        System.out.println(&quot;LockSupport park 之后……&quot;);

    &#125;

&#125;</code></pre>
<ol>
<li>线程阻塞，可以看出 permit 只能有一个</li>
</ol>
<h4 id="中断可以使-park-继续执行并不会抛出异常"><a href="#中断可以使-park-继续执行并不会抛出异常" class="headerlink" title="中断可以使 park 继续执行并不会抛出异常"></a>中断可以使 park 继续执行并不会抛出异常</h4><pre><code class="java">public class LockSupportTest &#123;
    public static void main(String[] args)  &#123;

        Thread thread = new Thread(new Runnable() &#123;
            @Override
            public void run() &#123;
                System.out.println(&quot;线程 &quot; + Thread.currentThread().getName() + &quot;开始执行 park&quot;);
                LockSupport.park(this);
                System.out.println(&quot;线程 &quot; + Thread.currentThread().getName() + &quot;执行 park 结束&quot;);

                System.out.println(&quot;线程 &quot; + Thread.currentThread().getName() + &quot;开始执行 park 第二次&quot;);
                LockSupport.park(this);
                System.out.println(&quot;线程 &quot; + Thread.currentThread().getName() + &quot;执行 park 第二次 结束&quot;);

            &#125;
        &#125;);

        try &#123;
            thread.start();
            // 保证 上面线程先执行，然后再主线程
            Thread.sleep(5000);
            System.out.println(&quot;开始执行 unpark(thread)&quot;);
            // LockSupport.unpark(thread);
            thread.interrupt();
            Thread.sleep(5000);
            System.out.println(&quot;执行 unpark(thread) 结束&quot;);
        &#125; catch (InterruptedException e) &#123;
            e.printStackTrace();
        &#125;

    &#125;
&#125;</code></pre>
<p>输出结果：</p>
<pre><code class="text">/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/bin/java ...
线程 Thread-0开始执行 park
开始执行 unpark(thread)
线程 Thread-0执行 park 结束
线程 Thread-0开始执行 park 第二次
线程 Thread-0执行 park 第二次 结束
执行 unpark(thread) 结束</code></pre>
<ol>
<li>可以看出线程中断，park 会继续执行，并且没有抛出异常。</li>
<li>thread.interrupt(); 调用之后， 设置线程中断标示，unpark 没有清除中断标示，第二个 park 也会继续执行。</li>
</ol>
<h4 id="使用诊断工具"><a href="#使用诊断工具" class="headerlink" title="使用诊断工具"></a>使用诊断工具</h4><pre><code class="bash">liuzhihang % &gt; jps
76690 LockSupportTest
77130 Jps
liuzhihang % &gt; jstack 77265
...
&quot;main&quot; #1 prio=5 os_prio=31 tid=0x00007f7f3e80a000 nid=0xe03 waiting on condition [0x000070000dfcd000]
   java.lang.Thread.State: WAITING (parking)
        at sun.misc.Unsafe.park(Native Method)
        at java.util.concurrent.locks.LockSupport.park(LockSupport.java:304)
        at com.liuzhihang.source.LockSupportTest.main(LockSupportTest.java:14)</code></pre>
<ol>
<li>中间省略部分，但是可以看出线程进入 <code>WAITING</code> 状态</li>
</ol>
<h3 id="源码分析"><a href="#源码分析" class="headerlink" title="源码分析"></a>源码分析</h3><pre><code class="java">
public class LockSupport &#123;

    private static final sun.misc.Unsafe UNSAFE;

    /**
     * 为线程 thread 设置一个许可
     * 无许可，则添加一个许可，有许可，则不添加
     * 如果线程因为 park 被阻塞， 添加许可之后，会解除阻塞状态
     */
    public static void unpark(Thread thread) &#123;
        if (thread != null)
            UNSAFE.unpark(thread);
    &#125;

    /**
     * 有许可，则使用该许可
     * 没有许可，阻塞线程，直到获得许可
     * 传递 blocker 是为了方便使用诊断工具
     */
    public static void park(Object blocker) &#123;
        Thread t = Thread.currentThread();
        setBlocker(t, blocker);
        UNSAFE.park(false, 0L);
        setBlocker(t, null);
    &#125;

    /**
     * 设置线程的 blocker 属性
     */
    private static void setBlocker(Thread t, Object arg) &#123;
        // Even though volatile, hotspot doesn&#39;t need a write barrier here.
        UNSAFE.putObject(t, parkBlockerOffset, arg);
    &#125;
&#125;</code></pre>
<p>LockSupport 的 park unpark 方法，实际调用的是底层 Unsafe 类的 native 方法。</p>
<pre><code class="java">
public final class Unsafe &#123;

    public native void unpark(Object var1);

    public native void park(boolean var1, long var2);
&#125;</code></pre>
<p>既然调用了 Unsafe 到此处肯定不能善罢甘休。</p>
<img src= "https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/loading.gif" data-lazy-src="https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/article/ffecad2c8644bab7d152be118c31722d.jpg" align=center />


<h4 id="hotspot-源码"><a href="#hotspot-源码" class="headerlink" title="hotspot 源码"></a>hotspot 源码</h4><p>这块是下载的官方包中的源码，阅读并查阅资料了解的大概逻辑，不清楚之处，希望指导出来。</p>
<p>也可以直接跳过直接看结论。</p>
<p>查看jdk源码<br><a target="_blank" rel="noopener" href="http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/5a83b7215107/src/share/vm/runtime/park.hpp">http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/5a83b7215107/src/share/vm/runtime/park.hpp</a></p>
<p>这块在以 os_linux 代码为例<br><a target="_blank" rel="noopener" href="http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/5a83b7215107/src/os/linux/vm/os_linux.cpp">http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/5a83b7215107/src/os/linux/vm/os_linux.cpp</a></p>
<img src= "https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/loading.gif" data-lazy-src="https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/article/c-parker.png" align=center />


<img src= "https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/loading.gif" data-lazy-src="https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/article/c-park.png" align=center />

<img src= "https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/loading.gif" data-lazy-src="https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/article/c-unpark.png" align=center />


<ol>
<li>在底层维护了一个 <code>_counter</code> 通过更新 <code>_counter</code> 的值来标示是否有证明。</li>
<li>在 park 时，判断 <code>_counter</code> 为 0，则阻塞等待，为 1 则获得更新为 0 并返回。</li>
<li>在 unpark 时，判断 <code>_counter</code> 为 0，则给予凭证，并唤醒线程，为 1 则直接返回。</li>
</ol>
<h3 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h3><p>总结也是和预想的是相同的。</p>
<ol>
<li>许可（permit）的上限是1，也就是说只有 0 或 1 。</li>
<li>park: 没有许可的时候，permit 为 0 ，调用 park 会阻塞；有许可的时候，permit 为 1 ， 调用 park 会扣除一个许可，然后返回。</li>
<li>unpark：没有许可的时候，permit 为 0 ，调用 unpark 会增加一个许可，因为许可上限是 1 ， 所以调用多次也只会为 1 个。</li>
<li>线程初始的时候是没有许可的。</li>
<li>park 的当前线程如果被中断，会立即返回，并不会抛出中断异常。</li>
</ol>
<h4 id="扩展"><a href="#扩展" class="headerlink" title="扩展"></a>扩展</h4><ul>
<li>park/unpark 和 wait/notify 区别</li>
</ul>
<ol>
<li>park 阻塞当前线程，unpark 唤醒指定线程。</li>
<li>wait() 需要结合锁使用，并释放锁资源，如果没有设置超时时间，则需要 notify() 唤醒。而 notify() 是随机唤醒线程。 </li>
</ol>
</article><div class="post-copyright"><div class="post-copyright__author"><span class="post-copyright-meta">文章作者: </span><span class="post-copyright-info"><a href="mailto:undefined">liuzhihang</a></span></div><div class="post-copyright__type"><span class="post-copyright-meta">文章链接: </span><span class="post-copyright-info"><a href="https://liuzhihang.com/2020/07/05/source-code-locksupport.html">https://liuzhihang.com/2020/07/05/source-code-locksupport.html</a></span></div><div class="post-copyright__notice"><span class="post-copyright-meta">版权声明: </span><span class="post-copyright-info">本博客所有文章除特别声明外，均采用 <a href="https://creativecommons.org/licenses/by-nc-sa/4.0/" target="_blank">CC BY-NC-SA 4.0</a> 许可协议。转载请注明来自 <a href="https://liuzhihang.com" target="_blank">程序员小航</a>！</span></div></div><div class="tag_share"><div class="post-meta__tag-list"><a class="post-meta__tags" href="/tags/%E6%BA%90%E7%A0%81%E7%AC%94%E8%AE%B0/">源码笔记</a><a class="post-meta__tags" 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href="#%E7%BA%BF%E7%A8%8B%E5%88%9D%E5%A7%8B%E6%9C%89%E6%B2%A1%E6%9C%89%E8%AE%B8%E5%8F%AF%EF%BC%9F"><span class="toc-text">线程初始有没有许可？</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#%E6%B7%BB%E5%8A%A0%E8%AE%B8%E5%8F%AF%E5%B9%B6%E6%B6%88%E8%80%97%E8%AE%B8%E5%8F%AF"><span class="toc-text">添加许可并消耗许可</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#%E8%AE%B8%E5%8F%AF%E4%B8%8A%E9%99%90%E4%B8%BA-1"><span class="toc-text">许可上限为 1</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#%E4%B8%AD%E6%96%AD%E5%8F%AF%E4%BB%A5%E4%BD%BF-park-%E7%BB%A7%E7%BB%AD%E6%89%A7%E8%A1%8C%E5%B9%B6%E4%B8%8D%E4%BC%9A%E6%8A%9B%E5%87%BA%E5%BC%82%E5%B8%B8"><span class="toc-text">中断可以使 park 继续执行并不会抛出异常</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#%E4%BD%BF%E7%94%A8%E8%AF%8A%E6%96%AD%E5%B7%A5%E5%85%B7"><span class="toc-text">使用诊断工具</span></a></li></ol></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E6%BA%90%E7%A0%81%E5%88%86%E6%9E%90"><span class="toc-text">源码分析</span></a><ol class="toc-child"><li class="toc-item toc-level-4"><a class="toc-link" href="#hotspot-%E6%BA%90%E7%A0%81"><span class="toc-text">hotspot 源码</span></a></li></ol></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E6%80%BB%E7%BB%93"><span class="toc-text">总结</span></a><ol class="toc-child"><li class="toc-item toc-level-4"><a class="toc-link" href="#%E6%89%A9%E5%B1%95"><span class="toc-text">扩展</span></a></li></ol></li></ol></div></div><div class="card-widget card-recent-post"><div class="item-headline"><i class="fas fa-history"></i><span>最新文章</span></div><div class="aside-list"><div class="aside-list-item"><a class="thumbnail" href="/2021/09/04/the_converter_converts_front_end_parameters_to_enumerations.html" title="使用 SpringBoot 转换器将前端参数转换为枚举"><img src= "https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/loading.gif" 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